This invention relates to a novel type of crane useful in many different environments but having particular usefullness in offshore applications. It overcomes problems which have long vexed the operators of offshore production facilities and marine drilling rigs of all types.
Offshore platforms need cranes to rapidly and safely load and off-load various material and personnel from floating vessels in the open sea to and from the fixed structures. The primary loads imposed upon such cranes are essentially of two types, a vertical load and an overturning moment. The vertical load in turn may be considered to consist primarily of two components, the dead weight of the crane structure itself and the actual load being lifted under dynamic conditions. It is to be realized that such conditions can be extremely dynamic, as, for example, when a vessel suddenly drops from the top of a wave to the bottom of a trough without adequate slack in the lines to compensate for such a rapid displacement. The dynamic loading of such cranes under such conditions can be, and often is, quite severe. The overturning moment is essentially the product of the dynamic load and the distance from the load to the centerline of rotation of the crane. This overturning moment is often applied impulsively.
Dockside cranes have long encountered similar conditions. The engineers of the eighteenth and nineteenth centuries attempted to resolve these problems by separating a pair of bearings or pivot points as widely as possible from each other. Perhaps because of the long-standing tradition with masts and riggings of sailing vessels, these early engineers separated these bearings vertically and resolved the overturning moment by a permanently mounted foundation fixed to the earth.
It eventually was realized that the utilization of these early cranes and derricks could be increased were they movable from place to place. The desire for such mobility presented two primary requirements which may be fairly said to have led directly to the configuration of the modern construction cranes which have been adapted for use in the offshore petroleum industry.
The first requirement for mobility was that such cranes could no longer be permanently attached to a foundation fixed to the earth. This in turn directly led to the use of counterweights to create an approximately equal but opposite overturning moment or couple to that created by the load, thus essentially reducing the loading on such mobile cranes to a vertical load on the wheels or tracks--in essence a balancing operation. It was soon realized that the actual weight or mass of the counterweight could be significantly reduced by causing it to rotate with the crane, thereby keeping it in the most advantageous position with respect to the load. It was also soon realized that the weight of the crane boom itself and any portion of the crane structure on the load side of the centerline of rotation significantly reduced the lifting capability of such cranes, thus spurring considerable effort to develop light weight and highly stressed boom structures, often of exotic materials.
The second requirement for mobility was a limitation on height to clear overhead obstructions, which precluded the use of a pair of vertically separated bearing assemblies. It then became necessary to resist the overturning moment by horizontally spaced bearings situated close together. Because such cranes typically must be capable of revolving 360.degree., such bearing arrangements typically took the form of a circle. The two methods in use today for this purpose are known as Hook Rollers and Ball Rings, with the latter sometimes being referred to also as Slewing Rings.
When offshore oil exploration beyond the sight of land was first accomplished around 1947, the only cranes available were construction cranes which had evolved as outlined above. These cranes had many shortcomings when removed from their intended application and transferred to offshore platforms to transfer material and personnel from floating vessels in the open sea. The balancing condition--or, more precisely, the impending loss of balance--could no longer satisfactorily be used to warn of impending overload situations when loading from a heaving vessel, a condition which frequently resulted in cranes being toppled into the ocean.
Mere removal of the undercarriage and permanent attachment of the rotating superstructure to the platform were only marginal improvements at best since impending unbalance could no longer be used as a `safety valve` when loading such cranes offshore. Designers necessarily had to strengthen such designs considerably in order for such cranes to have any chance at all of performing their intended functions, and the resulting cranes were extremely heavy, expensive and still unsatisfactory in operations.
A very few designers decided to design cranes specifically for the offshore industry and to be affixed permanently to offshore platforms. Since such cranes had no need for mobility, low height was no longer a requirement, and vertically separated bearing assemblies could again be employed. The affixable, pedestal-type crane with center post (or `king` post) removed both the requirement for counterweights and the impetus for light weight, exotic boom structures since such cranes were intended only for fixed mounting.
The pedestal-type, center post, affixable crane was a considerable improvement over the "ball ring" or "slewing ring" cranes, which generally require removal of the entire crane rotating structure from the slew ring and platform in order for the bearing to be replaced. Additionally, such designs generally combined the bearing function and structural function into a single mechanical assembly--functions which have incompatible if not mutually exclusive characteristics in that bearings need very hard materials which are inherently brittle while structural members need ductile characteristics in order to withstand the repeated shock loadings to which offshore cranes are subjected. The king post design, on the other hand, allows replacement of the swing bearings without the use of another crane, and thus was seen as a significant advance in the state of the art.
Despite its many advantages, and despite the greater design freedom permitted by separation of the bearing and structural functions, the bearings of the king post designs continued to pose problems. U.S. Pat. No. 4,061,230, for which applicant was a co-inventor, discloses a plurality of roller assemblies attached to the rotating superstructure of the crane and disposed about the king post. Each such roller assembly comprises a pair of small diameter, horizontal rollers pivotable about an apex displaced from the central post in order to permit the roller assemblies to adapt to irregularities in the central or king post. However, this reference does not contemplate nor teach a unitary structure or method for permitting ease of access to such bearings for inspection or removal. While this design and similar designs are in frequent use, they suffer from a number of disadvantages. Unless such rollers are of extremely small diameter in comparison to the center post, they will require a good bit of space, and if they are comparatively small, the rollers will frequently slide on the king post rather than rotate about their axles. This condition becomes even more pronounced when any grease or oil accumulates on either the rollers or their track around the post, which in turn may cause `flat spots` to wear on the rollers or cause the rollers to cut a groove in the post. The latter problem is frequently evident when the rollers are made of a material harder than that of the king post. Such a groove can lead to structural failure in the king post without warning, with the crane assembly falling from its mounting. Also, replacement of the rollers and/or roller assemblies is normally quite difficult because of the extremely tight space containing the same.
Attempts to overcome these problems directly led to a third generation of modern crane design. These designs generally affixed a removable wear strip to the center post and a mating ring to the rotating superstructure which slides on the stationary wear strip as the superstructure revolves about the king post. This concept is exemplified by U.S. Pat. No. 4,184,600 to applicant and another. While overcoming the problems of the multiple roller design and experiencing considerable commercial success, such designs are not themselves without disadvantages. The wear strips must of necessity be installed on the center posts before the superstructures are mounted, and the clearances therebetween must necessarily be quite small. Since such superstructures may be quite large and heavy objects, it is not always easy to maneuver them into place with the degree of precision required, particularly if the lifting crane is on a vessel. These factors result all too often in damage to or even destruction of the wear strips during installation of the superstructure over the king post. Additionally, such wear strips are quite difficult to install properly. Ordinarily the wear strips will not fit absolutely tightly around the center post, which can result in a bulge or wave in the strips as the crane is revolved. This in turn leads to premature failure of the wear strip fasteners, thus allowing the strips to slide about and be destroyed in short order.
Still another disadvantage of such a bearing design arises from the inevitable misalignment between the axis of rotation of the superstructure under load and the vertical axis of the center post. Although quite small, this angular misalignment causes the lower edge of the mating ring affixed to the rotating superstructure to tend to cut the stationary wear strip. While such bearings may be replaced with considerably less difficulty than those of previous designs, it is nevertheless a not insignificant inconvenience and expense to have to replace such bearings prematurely.
Attempts to overcome these disadvantageous features in turn led to the fourth generation of modern pedestal-type cranes as exemplified by U.S. Pat. No. 4,354,606 to applicant and another. This design utilizes removeable semicircular shoes mounted within the rotating superstructure to which the wear strips are then affixed. Since the wear strips need not be affixed prior to mounting the superstructure, and since the superstructure need only be centered about the center post as taught in U.S. Pat. No. 4,184,600 and not elevated as required by the '600 design, the damage or destruction to the wear strip during installation is eliminated. However, this design is also subject to angular misalignment between center post and superstructure, which causes extremely high point or line loading of the wear strips. This tendency toward point loading is exacerbated by the necessary difference between the inside diameter of the shoes with wear strips attached and the outside diameter of the center post, resulting in only a very small portion of the wear strips actually being in contact with the pedestal when under load, which in turn results in a relatively low load carrying capability for the shoes.
Owing to the "point" or "line" nature of the loading, the load carrying capability cannot be increased simply by the expedient of enlarging the bearing surface area: only a small fraction of the existing bearing surface area is actually utilizeable, and increasing the surface area of such bearings would only increase the amount of unused bearing area, and would not increase the load carrying capability at all. To increase the actual load carrying capability of such cranes, their designers greatly increased the separation between the upper and lower horizontal bearings. This in turn results in cranes which are `over tall` in relation to their moment-resisting ability and which are somewhat overweight when compared to similar capacity cranes of other designs. Heretofore, these height and weight penalties were not critical, but with growing concern about helicopter safety--and increasing regulations limiting approach angles to helipads--the allowable heights of platform equipment such as cranes are becoming more limited. Additionally, the increased quality controls placed on the industry have combined with the increasing price of steel to cause the costs of fabricated steel weldments such as center posts and rotating superstructures to increase radically in recent years.
Thus for safety reasons the industry is in urgent need of an improved crane design which can transmit larger actual bearing loads with a significantly reduced overall height and which can operate in the offshore environment without potentially catastrophic defects building up latently. In addition there is a pressing economical need for an improved design that will reduce the initial capital cost required and which can extend the intervals between bearing replacements with their associated high downtime costs.